This invention relates to arrangements for simultaneously generating sinusoidal signals at frequencies which are in a fixed relationship to each other, and to angle modulation data receivers for receiving frequency or phase modulated signals which use such signal generators to generate demodulating signals.
It is often desirable when transmitting signals modulated onto a carrier through a channel having substantial nonlinearities, as for example a channel including an amplifier subject to compression, to select a modulation method which maintains a constant carrier amplitude. Angle modulation is constant-amplitude modulation, and includes both frequency and phase modulation (FM and PM). The phase modulation techniques may include phase-shift keying (PSK), quadrature phase shift keying (QPSK) or offset QPSK (OQPSK).
Among the angle modulation techniques is frequency shift keying (FSK), which is the result of frequency modulating a carrier by means of bi-level digital signals. A bi-level digital information signal takes one of two states (logic high and logic low) depending upon the state of the digital information. According to one convention, a logic high level (HIGH) is called a MARK and a logic low level (LOW) is termed a SPACE. In frequency shift keying a MARK is represented by a first frequency and a SPACE by a second frequency. Minimum-shift keying or MSK is variant of FSK in which the deviation between the MARK and SPACE-representative modulated frequencies is one-half the bit rate. For example, for the case of a bit rate of 1200 bits-per-second (b/s), the deviation of the carriers is 600 Hertz (Hz), which corresponds to .+-.300 Hertz about the nominal carrier frequency. MSK modulation is advantageous because it is efficient in terms of the ratio of data rate to channel bandwidth (bits/sec/hertz). The carrier of an MSK-modulated signal is suppressed because there is no actual carrier at the nominal carrier frequency, and it is therefore not directly available to the receiver for generating a demodulating signal.
Because the MSK signal consists of MARK signals at frequency F.sub.1 and SPACE signals at frequency F.sub.2, it might appear that the frequency spectrum should include strong components of frequencies F.sub.1 and F.sub.2. The carrier signal for a string of symbols (MARK or SPACE) of the same type is a continuous signal at F.sub.1 and F.sub.2 and will in fact add in-phase to produce strong components. However, when the data stream is modulated by random symbols, the starting phase of the carrier for any symbol is established by the ending phase of the carrier for the preceding symbol. Since MSK carrier frequencies F.sub.1 and F.sub.2 are selected so that during each bit interval an even number of carrier cycles occur plus (F.sub.1) and minus (F.sub.2) a quarter carrier cycle, the phases of the two carriers rapidly reach a condition of bursts of in phase and quadrature phase carrier, which effectively suppresses the F.sub.1 and F.sub.2 frequencies. This suppression of the carrier is also characteristic of BPSK, QPSK or OQPSK and other phase shift keyed modulations.
Many demodulators of signals having a suppressed carrier apply the received signal to a frequency multiplying circuit to convert the data modulated signal to a frequency multiplied carrier frequency, which allows reconstruction of the carrier and thereby allows low noise coherent demodulation of a signal. Frequency doubling is used to recover the carrier of bi-phase modulated signals and of MSK signals, and frequency quadrupling of the signal is used to extract the carrier in the case of quadrature phase modulated signals.
In such receivers, the frequency doubled or frequency quadrupled carrier is maintained in a substantially constant phase relationship with a locally generated oscillator which is nominally at the same frequency as the frequency multiplied carrier. The actual demodulation of the signal is accomplished by a demodulating signal which is at the actual carrier frequency. Consequently, the actual demodulating signal is at a submultiple of the frequency of the locally generated oscillator signal which is locked to the frequency multiplied carrier of the received signal. Thus, the receiver includes a demodulating signal generator which is locked to a submultiple of the frequency of a higher frequency locally generated signal. The locking together of two oscillators which are at different frequencies is normally accomplished by a counter coupled to the higher frequency oscillator which counts down to the lower frequency for phase locking to the lower frequency oscillator by means of a phase lock loop (PLL). The narrow bandwidth typical of a PLL prevents rapid changes of phase of the oscillators such as may be required when adjusting the phase to generate demodulating signals for burst communications. A scheme is desirable in which signals at disparate frequencies may be generated coherently and in a manner such that rapid phase changes may be made to both signals.